The field of this invention shape-memory-effect actuators and in particular those usages of shape-memory alloy as they apply to making linear electro-mechanical actuators.
Shape-memory-effect (SME) alloys have been known and available for many years. Principal applications have used the nickel-titanium SME alloys in high-performance products such as aircraft hydraulic couplings. Because of their dramatic strength and response to temperature, SME alloys have continuously been proposed as alternatives to motors, solenoids, bimetallic or wax-type actuators. Although not a panacea, a SME approach to electro-mechanical actuation may offer advantages which conventional approaches would find difficult or impossible. For example, large amounts of recoverable strain available from SME alloys offer work densities up to ten times higher than conventional approaches. High electrical resistivity (similar to nichrome) permits direct electrical actuation without extra parts and with efficient use of available energy. Furthermore, large available material strains permit extremely long strokes, constant force during the stroke, and high starting force.
SME alloys have been used for actuator-type devices previously. Generally, the material is a nickel-titanium alloy called Nitinol.RTM. or Tinel.RTM. although copper-based alloys have been used in many similar applications. Applicant's copending U.S. Pat. Application Ser. No. 474,931, filed March 14, 1983, which is incorporated herein by reference, discloses various actuators employing a shape-memory alloy component. The instant invention is an improvement over that disclosed in applicants' above-mentioned application in that the instant actuator provides a reset mechanism that releases the actuator after it has retracted a specific distance and also interrupts the electrical circuit when the actuator is reset. The instant actuator is also provided with a self-protection means to protect the SME element from accidental and deliberate overloads, and to accommodate the extra motion required for high-cycle design life. An overload occurs during a jam of the actuator or when a load in excess of a predetermined amount designed into the actuator occurs.